refs #17601 [finc] Review offcanvas behaviour on XS

* For offcanvas=true:
** Set focus to back button on sidebar resp. to active element on account sidebar
** Add back button for Author search on result list
** Add toggle button for search tips
** Add back button for search tips
** Add toggle button on page Library Cards
** Add toggle button on page Change Password
* For offcanvas=false:
** Add back button
** Add link to the sidebar (toggle button)
** Add link to the content (back button)
** Set focus to back button on sidebar resp. to active element on account sidebar
* Add new translation for search tip toggle button
* adds aria-labels to offcanvas

* sets sidebar for XS
** sets default to offcanvas true
** sets default to bookbag true
** makes sidebar on XS 100% wide when offcanvas is off
** corrects various scss glitches

* use altered function in separate JS file
** avoid copy-paste overhead in case of changes on parent theme